Large traction vehicles such as locomotives are typically powered by DC electric traction motors coupled to axles of the vehicle. For example, a locomotive commonly has four or six wheel and axle sets per vehicle, with each set being connected via appropriate gearing to the drive shaft of an electric motor, referred to in the art as a traction motor. Traction motors, when operable, are supplied with electric power from a controlled source, commonly a traction alternator driven by the locomotive's engine. The traction motors apply torque to the locomotive's wheels, which in turn exert tractive effort on the rails on which the locomotive is travelling. The DC traction motors can also be reconfigured to apply braking effort which is then used to either control speed or to reduce speed when stopping, i.e. to perform braking. This function is referred to as dynamic braking.
Many diesel electric locomotives in operation today are equipped with dynamic braking, whereby the locomotive's traction motors, primarily used to power its wheels, are reconfigured to become generators used to slow the locomotive down when braking is required. As the locomotive slows down below a critical speed, the braking effort of the traction motor is reduced to the point where it becomes ineffective. A traditional method of overcoming this outcome is to short out portions of the braking resistors, allowing the current to rise, thereby increasing braking effort. During the transition of shorting out portions of the dynamic braking grid, there is a temporary reduction of braking effort. At certain speeds, due to the specific nature of the locomotive's control system, it has been found that there is a below optimum braking effort.
It is an object of the following to address the above-described disadvantages.